The present invention relates to an improved rendering technique for halftone images and corresponding improved materials, used for generating a screened reproduction of a continuous tone image by means of an electronic screening modulation of the original. An example of the technique is given for a thermal recording material.
Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
Direct thermal thermography is concerned with materials which are substantially not photosensitive, but are sensitive to heat or thermosensitive. Imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material.
Most of the xe2x80x9cdirectxe2x80x9d thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and an image is produced by a change of the local optical density.
U.S. Pat. No. 5,424,182 discloses a thermal imaging material and preparation.
Many reproduction methods are only capable of reproducing a small number of stable image tones. For example, offset printing is only capable of printing two stable tone values i.e. deposit ink or not. In order to reproduce images having continuous tones, a halftoning or screening technique is used. In the graphic arts environment, halftoning techniques convert density values of tints and images into a geometric distribution of binary dots that can be printed. The eye is not able to see the individual halftone dots, and only sees the corresponding xe2x80x9cspatially integratedxe2x80x9d density value. In a more general context, halftoning techniques can be seen as methods to convert xe2x80x9clow spatial, high tonal resolution informationxe2x80x9d into an equivalent of xe2x80x9chigh spatial, low tonal resolution informationxe2x80x9d. The qualifiers xe2x80x9clowxe2x80x9d and xe2x80x9chighxe2x80x9d have to be seen on a relative scale in this context. In amplitude-modulation screening the halftone dots, that together give the impression of a particular tone, are arranged on a fixed geometric grid. By varying the size of the halftone dots, the different tones of images can be simulated. Consequently, this technique can also be called xe2x80x9cdot-size modulation screeningxe2x80x9d. This class of halftoning technique is traditionally used in combination with a digital film recorder. A typical digital film recorder employs a scanning laser beam that exposes a photosensitive material at high resolution. The xe2x80x9cgridxe2x80x9d that defines the resolution at which the laser beam can be switched on or off, usually has an element size in the range of {fraction (1/1800)} of an inch. The photosensitive material can be a photographic film from which a printing plate is later prepared by means of photomechanical techniques. The smallest addressable unit on a recorder is often called a xe2x80x9cmicro dotxe2x80x9d, xe2x80x9crecorder elementxe2x80x9d, or xe2x80x9crelxe2x80x9d. Its size is referred to as the recorder xe2x80x9cpitchxe2x80x9d. As illustrated in FIG. 3a, a dot-size modulated halftone dot is made up of a clustered set of recorder elements.
The most important characteristics of a screening or halftoning technique for faithfully reproducing continuous tone information include:
1) The image rendering characteristics, more specifically the capability of the technique to render spatial detail in the original image content without the introduction of artifacts such as moirxc3xa9, textures and noise, as well as the capability to render a full range of tones;
2) The photomechanical characteristics of the halftone dots produced by the method, which determine how consistently halftone dots can be recorded, copied or duplicated in the different steps of the photomechanical preparation of the printing plates; and,
3) The behaviour of the halftones on an offset printing press.
The class of amplitude-modulation halftoning, with some of its variants, will now be reviewed in the light of the above characteristics, and their advantages and disadvantages will be discussed.
Amplitude-modulation screening has as its major advantages that it has excellent photomechanical reproduction characteristics, and that, for screens with rulings up to 200 lines/inch, it prints predictably on offset presses. An important disadvantage of amplitude-modulation screening, however, is the fact that unwanted patterns can occur within the halftoned image. Depending on their origin, these patterns are called subject moirxc3xa9, colour moirxc3xa9 or internal moirxc3xa9. Subject moirxc3xa9 results from the geometric interaction between periodic components in the original subject matter and the halftone screen itself. Methods addressing subject moirxc3xa9 are disclosed in e.g. U.S. Pat. No. 5,130,821, EP-A-0 369 302 and EP-A-0 488 324. These methods do not, however, completely solve the problem.
Colour moirxc3xa9 results from interferences between the a halftones of the different colour separations of the image. The use of screen angles for the different colour separations shifted by 60 degrees one with respect to the other has been suggested to address this problem. Several disclosures relate to the problem of generating screens with these angles or close approximations thereof. See e.g. U.S. Pat. Nos. 4,419,690, 4,350,996, 4,924,301 and 5,155,599. Other combinations of angles, frequencies or relative phases of the halftone dot patterns for the different colour separations have also been used to overcome the same problem, as described e.g. in U.S. Pat. Nos. 4,443,060, 4,537,470 and EP-A-0-501 126.
Internal moirxc3xa9 refers to patterns resulting from the geometric interaction of the halftone screen with the addressable grid on which they are rendered. Methods to reduce internal moirxc3xa9 are usually based on the introduction of a random element that breaks up or xe2x80x9cdiffusesxe2x80x9d the phase error that periodically builds up as a consequence of the frequency and angle relation between the halftone screen and the addressable grid on which it is rendered. Examples of such techniques are disclosed in U.S. Pat. Nos. 4,499,489, 4,700,235, 4,918,622, 5,150,428 and WO-A-9 004 898.
EP-A-0 734 147, EP-A-0 774 857 and EP-A-0 734 148 provide methods for generating screened reproductions of continuous tone images with improved reproduction characteristics.
These three patent applications disclose methods for improving the image quality by complicating the modulation of the laser. These solutions not only tend to complicate the manufacturing process, they are also time-consuming and costly.
Another screening technique is referred to as frequency modulation halftoning. Whereas in amplitude modulation screening the halftone dots, that together give the impression of a particular tone, are arranged on a fixed geometric grid and are their size is varied to simulate the different tones of images, in frequency modulation screening the distance between the halftone dots is modulated rather than their size. The arrangement of exposed microdots according to frequency modulation screening is shown in FIG. 3b. According to FIG. 3b frequency-modulation halftone dots consist of individual recording elements. Each black square in FIG. 3b represents one frequency-modulation halftone dot, occupying exactly one xe2x80x9crelxe2x80x9d. The whole FIG. 3b shows a region or set of dispersed black frequency-modulation halftone dots, covering each one individual recording element. In the prior art methods, the size of a frequency-modulated halftone dot equals the size of a recorder element.
Various frequency-modulation screening techniques have been disclosed and they can be divided into the following subclasses:
(1) Point-to-point thresholding based techniques;
(2) Error Diffusion techniques (and their variations); and,
(3) Special techniques, such as that disclosed in DE-A-2 931 098, and further developed in U.S. Pat. No. 4,485,397. The most representative example of point-to-point thresholding is the halftoning based on the xe2x80x9cBayerxe2x80x9d dither matrix. See BAYER, B. E. xe2x80x9cAn Optimum Method for Two-level Rendition of Continuous-tone Picturesxe2x80x9d, New-York: Proc. IEEE International Conference on Communications, Conference Record, 1973, pp. (26-11), (26-15). The Bayer dither matrix has a size that is a power of two, and contains threshold values that are arranged in such a fashion that, when thresholded against increasing levels of density, every halftone dot is xe2x80x9cas far away as possiblexe2x80x9d from the halftone dots that are used to render the lower density levels.
Another point-to-point thresholding technique uses a xe2x80x9cblue-noise maskxe2x80x9d instead of a Bayer dither matrix. It is described in U.S. Pat. No. 5,111,310. The blue-noise mask is the result of an optimization (filtering) performed iteratively (for the subsequent threshold xe2x80x9clayersxe2x80x9d) between the halftone dot patterns produced by the mask and their Fourier transforms.
The halftone dot patterns produced by the Bayer dither matrix contain strong periodic components, visible as xe2x80x9ctexturexe2x80x9d that can potentially create moirxc3xa9 problems similar to the dot-size modulation algorithms. Because the energy of the periodic dither components is xe2x80x9cspreadxe2x80x9d over the different harmonics, and because most of these harmonics have a relatively high frequency compared to the fundamental frequency of dot-size modulation, the moirxc3xa9 that occurs is less disturbing.
The xe2x80x9cBlue-noise maskxe2x80x9d threshold matrix produces distributions of halftone dots that are a periodic. This method is therefore free of the moirxc3xa9 problems that occur with the dot-size modulation methods or with the Bayer dither matrix. The aperiodic character of the halftone dot distributions of the blue-noise mask technique translates in the frequency domain into a xe2x80x9ccontinuousxe2x80x9d power spectrum. This suggests that at least some energy is also present in the very low frequency bands of the spectrum. This energy at low (visible) spatial frequencies is one of the reasons why tints rendered with the blue-noise mask technique can appear grainy. The relation between xe2x80x9cgraininessxe2x80x9d introduced by frequency-modulation halftoning methods and the shape of the frequency spectrum is extensively discussed by Ulichney, R. xe2x80x9cDigital Halftoningxe2x80x9d, Cambridge Mass.: MIT Press, 1987.
Perhaps the best known of all xe2x80x9cfrequency-modulationxe2x80x9d techniques is the error diffusion algorithm. It comes in many variations, but the principle is always the same: the error that occurs as a result of the binarization (or, in a more general context, the quantization) of the image data during the rendering is xe2x80x9cdiffusedxe2x80x9d to one or more of the unprocessed pixels. Best known is the Floyd and Steinberg algorithm (Floyd, R. W. and Steinberg, L. An Adaptive Algorithm for Spatial Greyscale, Proc. SID,1976. vol. 17/2, pp. 75-77). However many variations exist that usually differ in the order in which pixel halftoning is done, in how the error is diffused (to how many pixels and with which weights), or in how a random element is introduced in the algorithm to break up the unwanted patterning that can occur with some of them.
All of the frequency-modulation halftoning techniques that produce aperiodic halftone dot distributions share the advantage that they are much less sensitive to the problems of moirxc3xa9 as compared to the xe2x80x9cdot-size modulationxe2x80x9d techniques.
A problem associated to frequency modulated halftone images is their noisy or grainy appearance to the observer. This problem is less conspicuous for amplitude modulated halftone images, since according to that technique the halftone dots are laid out on a periodic grid, which is not disturbing for the human observer as long as the screening frequency is at least 100 lpi (lpi=lines per inch, 1 inch is 25.4 mm). Since the halftone dots according to frequency modulated halftoning are located more or less randomly, an image reproduced by this technique has a broad spatial frequency spectrum. A spacial frequency spectrum may be obtained by applying a two-dimensional discrete Fourier transform to the screened image data. Although it seems that frequency modulated halftone dots, due to their small size and small relative distance, would introduce only high frequencies in the spatial frequency spectrum, a discrete two-dimensional Fourier transform of such a halftone image shows that also low frequencies are present and have a considerable energy level in the spectrum. The high frequencies in the spectrum are filtered by the human eye and are not disturbing for the human observer. The noisy appearance to the human observer of frequency modulated halftone dots can be attributed to the presence of these low frequency components. This results in a degradation of the image quality in printing systems, even for printing systems having a relatively high spatial resolution for producing frequency modulated halftone dots.
In order to improve the image quality of thermal systems the manufacturers of the thermal printers have focused on fine tuning the heating controls of the thermal printhead.
WO-A-95 012 493 discloses a micro-addressing technique in direct thermal hyperacuity printers allowing high-resolution dot outline positioning. A driver circuit provides a plurality of drive energies having a plurality of thermal distributions associated herewith, producing non-periodic halftone dot distributions. These thermal distributions interact with print media to form binary images having active and inactive areas. A frequency modulated halftone image printed by such a printer still suffers from high noise levels, if no special measures are taken.
WO-A-95 033 330 discloses a method for local resolution enhancement by modulating the stream based on the previous, current and next grey scale values. Also this method does not address the image quality, perverted by the noise problem.
It is an object of the present invention to improve the image quality in printing systems.
It is a specific object of the invention to achieve better tone rendering.
It is another specific object of the invention to achieve less noise in screening applications, especially for low resolution printing systems.
The above-described deficiencies are addressed by a method according to the present invention for rendering a reproduction of a continuous tone image on a recording material. In this method, the continuous tone image are screened by a frequency-modulating halftone technique to obtain frequency modulated halftone data. The recording material is then scanned according the frequency modulated halftone data. The recording material shows a negative cross-talk effect upon adjacent exposed microdots.
A halftone image is an image in which the continuous tones or grey-levels of an original image are reproduced by varying the size of the halftone dots, the number of halftone dots per unit area and/or the density of the halftone dots in the pattern that forms the image. The density variation of halftone dots in a halftone image is coarser than the density or intensity variation of the corresponding original continuous tone image. In a binary printing system, the halftone dots can take two density levels: a lower density level and a higher. In multilevel printing systems, the halftone dots, or even each individual microdot of a halftone dot, may take one of more than two different density levels. The concept of multilevel frequency modulated halftoning is further explained in EP-A-0 682 438.
The terms frequency-modulation halftoning or FM-screening shall mean any halftoning technique in which: the average distance between the halftone dots changes as a function of the tone value; or the number of the resulting halftone dots per unit area varies with the tone value. A halftone dot is an area having a substantially constant first optical density, surrounded by an area having a substantially constant second optical density, different from the first optical density. A halftone dot may be formed by one black microdot on a white background. A halftone dot may also be formed by two or more adjacent black microdots on a background of white microdots, i.e. surrounded by white microdots; this is typical for regions having an integrated optical density, that is lower than a specific value. Alternatively, a halftone dot may comprise one or more white microdots, surrounded by black microdots; this is typical for regions having an integrated optical density, higher than a specific value. Halftone dots may also be defined on a substrate having ink accepting and ink repellant zones. A halftone dot may then be defined by an ink repellant zone surrounded by an ink accepting zone or vice versa.
The term negative crosstalk can be defined as a local microdot shrink effect, that appears in halftoning, when neighbouring microdots are exposed to energy for changing the optical density of the microdots. When the total surface area of two exposed adjacent microdots is smaller than the total surface area of two individual exposed microdots, the effect can be defined as negative crosstalk. Two microdots are adjacent, if the distance between their respective centres is smaller than the largest size of an isolated exposed microdot. If the shape of the microdot is circular, the size is its diameter; if the shape is square or rectangular, the size is the length of one diagonal; if the shape is elliptical, the size is the length of the longer axis of the ellipse, etc.
Also when two adjacent microdots are exposed and when the resulting optical density close to their touching border is considerably different from the optical density of the middle of the individual exposed microdots, this is referred to as negative cross-talk. For black exposed microdots, the touching border is white, for white exposed microdots, the touching border is black.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings.